If the focal-film distance changes, what must be recalculated to maintain correct exposure?

• A. kV
• B. FFD
• C. Exposure time
• D. mAs

Answer: (D)

When the focal-film distance changes, the amount of radiation reaching the film changes due to the inverse square law: doubling the distance reduces exposure to about a quarter. To keep the same receptor exposure, you must adjust the total X-ray output, which is the mAs (tube current times exposure time). Kilovoltage (kV) mainly affects beam quality and penetration, not the amount reaching the film, so it isn’t the factor you recalculate to maintain exposure. The distance itself (FFD) is what changes, but you don’t recalculate the distance—you adjust the exposure factors. Since exposure at the film is governed by mAs, the correct adjustment is to the mAs.